Learning science glossary: retrieval practice, spacing, interleaving and desirable difficulties

Definitions summarised from published reviews, studies and a chapter by Elizabeth and Robert Bjork, checked October 30, 2026. Course applications are our suggestions. Our editorial method

Desirable difficulties and the idea behind them

Elizabeth and Robert Bjork of UCLA use desirable difficulties for conditions of study and practice, such as spacing, interleaving, contextual variation and retrieval practice, that pose challenges but often enhance learning and transfer; Robert Bjork coined the term in 1994. They stress that the word desirable matters: if a learner lacks the background knowledge to respond to a difficulty successfully, it becomes an undesirable one.

The Bjorks explain why through two kinds of memory strength from their new theory of disuse (1992). Storage strength is how well learned and interconnected something is; retrieval strength is how easily you can access it right now. The higher an item's current retrieval strength, the smaller the gain in storage strength from restudying or retrieving it, so some forgetting between sessions makes the next review count for more. That is why the feeling of fluency is a poor guide to what you will remember.

Term	Plain meaning	Use it in a course
Desirable difficulty	A harder way to practise that pays off later (Bjork, 1994)	Choose the quiz over the summary video when both are offered
Storage strength	How well learned something is	Judge progress by delayed self-tests, not by how familiar a lesson feels
Retrieval strength	How accessible something is right now	Expect it to fade between sessions; that fade is when review helps most
Fluency illusion	Mistaking ease of rereading for learning; restudy raised confidence without improving delayed recall (Roediger and Karpicke, 2006)	Do not stop reviewing because the notes look familiar

Testing and retrieval terms

Roediger and Karpicke (2006, Psychological Science, DOI 10.1111/j.1467-9280.2006.01693.x) gave students either repeated study or recall tests without feedback. Restudy won on a test 5 minutes later, but testing produced substantially greater retention at 2 days and 1 week. Dunlosky and colleagues' 2013 review (DOI 10.1177/1529100612453266) rated practice testing as high utility. The Institute of Education Sciences 2007 practice guide rated quizzing to re-expose students to key content as strong evidence.

Term	Plain meaning	Use it in a course
Retrieval practice	Pulling information out of memory rather than putting it back in by rereading	Close the video and write the main points before checking
Testing effect	The finding that a recall test improves later retention more than restudying (Roediger and Karpicke, 2006)	Retake ungraded quizzes a few days later, not just once
Practice testing	Self-testing or practice quizzes, rated high utility by Dunlosky et al. (2013)	Turn each module heading into a question and answer it cold
Pre-questions	Questions asked before a topic is taught; the IES guide rates the evidence as minimal	Try the end-of-module quiz first if it is ungraded, then study
Generation	Producing an answer from memory yourself; the Bjorks call retrieving a more powerful learning event than restudying, provided you have the prior knowledge to attempt it	Predict the next step of a worked example before the instructor shows it

Timing and ordering terms

Cepeda and colleagues' 2006 meta-analysis in Psychological Bulletin (DOI 10.1037/0033-2909.132.3.354) pooled 839 assessments from 317 experiments and found that the gap between study sessions that gave the best retention grew as the time until the final test grew. Kornell and Bjork (2008, DOI 10.1111/j.1467-9280.2008.02127.x) found that mixing paintings by different artists improved learners' ability to identify new paintings by those artists, yet participants rated massed (blocked) study as more effective, even after their own test results showed the opposite.

Term	Plain meaning	Use it in a course
Spacing effect	Spreading study over several sessions beats cramming the same total time	Schedule two short reviews of each module in later weeks
Distributed practice	The technique form of spacing; rated high utility by Dunlosky et al. (2013)	Split a 3-hour weekend block into three 1-hour sessions
Lag (inter-study interval)	The gap between sessions on the same material	Use longer gaps for an exam months away, shorter ones for next week
Interleaving	Mixing different problem types or categories instead of doing one type in a block	Make a mixed problem set from three earlier modules
Blocked practice	Doing all problems of one type together; feels easier and is often judged better by learners	Fine for a first pass, then switch to mixed sets
Varied practice	Changing the conditions of practice instead of keeping them constant; the Bjorks warn that unchanging settings supply cues that will not be there on an exam	Apply a formula or query to a second dataset, not just the course one

Explanation and representation terms

The IES practice guide (Pashler and colleagues, 2007) rated asking deep explanatory questions as strong evidence, and rated interleaving worked examples with problems, combining graphics with words, and connecting abstract and concrete representations as moderate. Dunlosky et al. rated elaborative interrogation and self-explanation as moderate utility, and rated highlighting, rereading and summarisation as low.

Term	Plain meaning	Use it in a course
Elaborative interrogation	Asking why a fact is true; moderate utility (Dunlosky et al., 2013)	Add a "why does this work?" line under each key point in your notes
Self-explanation	Explaining each step of a solution to yourself; moderate utility	Narrate a code walkthrough line by line before running it
Worked example	A fully solved problem studied before solving similar ones; IES moderate evidence when alternated with practice	Alternate: one worked example, then one problem you solve alone
Graphics with words	Pairing a diagram with a verbal description; IES moderate evidence	Sketch the diagram from memory and label it in words
Concrete and abstract links	Connecting a general rule to specific cases; IES moderate evidence	For each formula, write one real example from your own work
Delayed judgment of learning	Rating how well you know something after a delay, not right after study; IES minimal evidence	Decide what to review a day later, not straight after the lesson

How to use this glossary

These findings come mostly from laboratory and classroom studies of particular materials. They are well replicated as general principles, but how large the benefit is for your course will vary.

Start with the two high-utility techniques: practice testing and distributed practice.
Add interleaving once you have covered at least two or three related topics.
Treat feelings of familiarity as retrieval strength, not storage strength; check with a delayed self-test.
Use low-utility habits (highlighting, rereading) only as a lead-in to testing, not as the main activity.
Our method guides turn these terms into schedules: retrieval practice for notes, spaced practice calendars, interleaved problem sets and worked-example alternation.

Questions before you decide

What are desirable difficulties?

They are study conditions that make practice feel harder but often improve long-term learning and transfer, such as spacing, interleaving, varying practice conditions and retrieval practice. Robert Bjork coined the term in 1994, according to Bjork and Bjork.

What is the difference between retrieval practice and the testing effect?

Retrieval practice is the activity of recalling information from memory. The testing effect is the research finding that doing so improves later retention more than restudying, as Roediger and Karpicke showed at 2-day and 1-week delays.

Which study techniques have the strongest evidence?

Dunlosky and colleagues' 2013 review rated practice testing and distributed practice as high utility, elaborative interrogation, self-explanation and interleaved practice as moderate, and highlighting, rereading and summarisation as low.

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